1. Field of the Invention
The present invention relates to devices for displaying characters on a picture screen of a display unit in a line by line manner and in particular to such a device in which an image repetition memory is periodically read out and which employs an image generator containing a character generator for generating data words corresponding to the shapes of the characters.
2. Description of the Prior Art
An apparatus for displaying characters on a picture screen of a display unit is disclosed in German Pat. No. 2,540,734 in which characters to be displayed are encoded and stored as code characters in a data memory. An image control unit removes the code characters from the data memory in a periodic sequence corresponding to 50 or 60 image changes per second. The image control unit contains a character generator in which data words corresponding to the shapes of the characters which can be displayed on the picture screen are stored. The corresponding data words are read out from the character generator as a function of the characters to be displayed on the picture screen and are transmitted to the display unit as line signals. If the display unit is a cathode ray tube, the line signals represent video signals with which a trace unblanking of the electron beam is undertaken.
A further apparatus for displaying characters is known from German Pat. No. 24 46 048 in which the image repetition memory is designed as a shift register. The code characters corresponding to the characters to be displayed constantly circulate in the shift register in accord with the picture frequency. By the use of a multiplexer the code characters respectively representing one line are read out from the shift register. The code characters are supplied to a character generator which emits data words corresponding to the shapes of the characters. In this known arrangement, as in the apparatus described above, video signals for a display unit which is a cathode ray tube are generated by means of the data words.
In conventional devices such as those described above, a line scanning pattern which is formed from a plurality of lines and which cannot be altered is necessary for the representation of the characters. Moreover, the characters can only be displayed at prescribed character locations. While suitable for many applications, known devices result in a relatively inflexible display pattern.